1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices and more particularly to unidirectional SAW devices driven by a single-phase drive source and which also have a broad passband.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Surface acoustic wave devices known as SAW devices have many uses, primarily in the UHF and VHF frequency ranges. SAW devices have been especially useful as impedance elements, resonators, and bandpass filters in these frequency ranges. Typical SAW devices have a substrate with at least a surface layer of piezoelectric material and surface acoustic wave transducers in interdigitated form disposed on the piezoelectric surface. The transducers convert an electric signal to surface acoustic waves propagating on the piezoelectric surface and vice versa.
SAW devices are compact, lightweight, robust, and, because they use planar technology, are economical to manufacture. They can be mass-produced using the same techniques developed so successfully for the production of silicon integrated circuits. A wide variety of analog signal processing functions can be achieved with SAW devices. Among other applications, they are currently used in pulse compression radar systems as receiver bandpass filters, or as resonators for stabilizing oscillators in numerous applications. They have replaced many of the coils, capacitors, and metal cavities of conventional radio frequency systems, removing the need for hand alignment and dramatically improving the reliability and performance. They have simultaneously resulted in significant reductions in both size and cost.
However, several problems are associated with the prior art surface acoustic wave transducers. One of the problems occurs because the transducer electrodes cause internal reflections which distort the transducer output and the shape of the input conductance. Another problem occurs when the transducer is used in filter applications. Triple transit distortion is caused by regeneration reflections between the transducers.
In order to eliminate triple transit distortion, three-phase and single-phase prior art devices are used to cause a greater amount of radiation in one direction in the crystal than in the reverse direction and thus form unidirectional transducers. In one configuration proposed for a single-phase unidirectional transducer (SPUDT), a device such as that disclosed in U.S. Pat. No. 4,353,046 is constructed in which the acoustic reflections are used to cancel the regenerative reflections and unidirectional behavior results. These transducers are simple to fabricate and tune and thereby overcome some of the disadvantages of the multiphase devices. However, in these devices the finger and gap widths in a single-phase unidirectional transducer (SPUDT) were typically of split-finger construction and were one-eighth (1/8) of the operating acoustic wavelength, thus limiting the frequency range of the device by photolithographic constraints to a maximum frequency of operation when compared to the simplest form of SAW transducer using quarter-wavelength or .lambda./4 electrodes.
With .lambda./4 electrodes, the reflectivity is unaffected by energy storage as the contributions from the front and back edges cancel. However, at other electrode widths they can significantly affect the value of the reflectivity. Unfortunately, for electrode widths of less than .lambda./4, the energy-storage reflections are generally of opposite phase to those resulting from the impedance discontinuities. The result is a substantial reduction in the electrode reflectivity for electrode widths less than .lambda./4.
Another two-level SPUDT configuration is set forth in U.S. Pat. No. 4,902,925, incorporated herein by reference in its entirety. This structure, commonly known as the "Hopscotch", employed a group-type sampling with all electrode widths at .lambda./4. Like the original SPUDT configuration, the first level of the "Hopscotch" transducer by virtue of the electrode groupings has no net internal reflections. Unidirectionality is only achieved by the addition of a second level metalization with this structure. However, since the electrode widths are .lambda./4, rather than .lambda./8 as in the original split-finger structure, greater internal reflectivity levels are achieved. Unfortunately, as a result of the sparse group type sampling the structure, the effective coupling is substantially reduced. The latter severely limits the maximum bandwidth or minimum insertion loss achievable with this transducer. In addition, this structure has significant group responses not far below the passband.
Independent from the "Hopscotch" transducer, another concept for a SPUDT was proposed which relies on unique crystal orientations as set forth in U.S. Pat. No. 4,910,839, incorporated herein by reference in its entirety. On these unique crystal orientations, a simple two-electrode-per-wavelength transducer exhibits unidirectional characteristics. Unfortunately, on these natural SPUDT (NSPUDT) orientations, the sense of directionality is determined by material properties of the crystal substrate and overlay material rather than by the transducer configuration as with other approaches. Consequently, reversing the sense of the directivity of the transducer is difficult.
Still another approach set forth in U.S. Pat. No. 5,073,763 relates to GSPUDT structures with 3/8.lambda. and 5/8.lambda. sampling. Reflectionless or unidirectional transducers and broad-band notch elements are all implementable with these configurations. The GSPUDTs disclosed in this patent are similar to the conventional SPUDT (CSPUDT) and the "Hopscotch" transducers in that single-level versions are reflectionless. Unidirectional characteristics are obtainable only from the two-level structure, but: (1) they can be made unidirectional on either CSPUDT or NSPUDT crystal orientations (2) they can be made unidirectional in either the forward or reverse directions by a simple change to the second level metalization; (3) they have greater coupling than many other types of transducers; and (4) they have smaller geometry than other transducers. Thus, this type of transducer can be used in combination with an NSPUDT transducer to implement low-loss filters, resonators, or the like.
These SAW transducers have a pattern of interdigitated electrodes on a piezoelectric substrate. The electrodes lie on the substrate on either a 3/8.lambda. or 5/8.lambda. grid such that adjacent electrodes have either a center-to-center spacing of 3/8.lambda. or a center-to-center spacing of 5/8.lambda.. The electrodes do not have to be physically located at every 3/8.lambda. or 5/8.lambda.. However, it is important to have a 3/81 or 5/8, group-type sampling which means that even if the transducer is withdrawal weighted, the remaining electrodes are always centered on the 3/8.lambda. or 5/8 grid with any adjacent electrodes having either a 3/8.lambda. or 5/8.lambda. center-to-center spacing depending upon whether the transducer is a 3/8.lambda. structure or a 5/8.lambda. structure.